Filters can be made of fine fiber webs of polymer materials. Polymer webs can be made by electrospinning, extrusion, melt spinning, air laid and wet laid processing, etc. The manufacturing technology of filter structures is vast for obtaining structures that can separate the particulate load from a mobile fluid stream. Such filtration media include surface loading media and depth media in which these media can be produced in a variety of geometric structures. Principles relating to the use of such media are described in Kahlbaugh et al., U.S. Pat. Nos. 5,082,476; 5,238,474; 5,364,456 and 5,672,399. In any filter structure containing any arbitrary selected filtration medium, the filter must remove a defined particle size, and at the same time, have sufficient lifetime to be economically justifiable in its particulate removing properties. Lifetime is generally considered to be the time between insulation and the time a filter obtains sufficient particulate load such that the pressure drop across the filter is greater than a predetermined level. An increased pressure drop can cause filter bypass, mechanical filter failure, fluid starvation, or other operating problems.
Nanofiber filter media have fueled new levels of performance in air filtration in commercial, industrial and defense applications where efficiency requirements have been low in comparison to HEPA (High Efficiency Particle Air) or ULPA (Ultra Low Penetration Air) levels.
Recent advancements in the nanofiber enhanced filtration field have extended the usability of nanofibers into applications with higher filtration efficiencies. In particular, these nanofiber matrices provide comparable performance to other commercially available HEPA media composed of sub-micron glass or expanded-PTFE membranes. Such performance is shown along with benefits of the technology. Nanofiber filter media is a viable solution in high efficiency applications with strict performance requirements.
A web can comprise a substantially continuous fiber mass and dispersed in the fiber a fiber spacer, spacer particulate or web separation means. The spacer or separation means causes the fiber web to attain a structure, in which the fiber mass or web portion, even though filled with particulate, has reduced solidity, separated fibers or separated web portions within the structure, increased the depth of the fiber web without increasing the amount of polymer or the number of fibers within the web. The resulting structure obtains improved filtration properties such as resistance to increased pressure drop, improved Figure of Merit, permeability and efficiency. Filtration efficiency is the characteristic of the filtration media that is related to the fraction of the particulate removed from the mobile stream. Efficiency is typically measured by a set test protocol defined below.
Surface loading filter media often comprise dense mats of fiber having a non-woven structure that are placed across the path of a mobile fluid stream. While the mobile fluid stream passes through the structure of the formed non-woven fibers, the particulate is typically removed from the streams at the filter surface with a certain efficiency and remains on the surface. In contrast to surface loading structures, depth media typically include a relatively (compared to surface loading media) thick structure of fiber having a defined solidity, porosity, layer thickness and efficiency. Depth media and in particular, gradient density depth media are shown in Kahlbaugh et al., U.S. Pat. Nos. 5,082,476; 5,238,474 and 5,364,456. In general, depth media act in filtration operations by impeding the particulate loading in a mobile fluid stream within the filter layer. As the particulates impinge the depth media fibrous structure, the particulate remains within the depth media and typically distributed onto and held with internal fibers and throughout the filter volume. In contrast, surface loading media typically accumulate particulate in a surface layer.
Groeger et al., U.S. Pat. No. 5,486,410, teach a fibrous structure typically made from a bicomponent, core/shell fiber, containing a particulate material. The particulate comprising an immobilized functional material held in the fiber structure. The functional material is designed to interact with and modify the fluid stream. Typical materials include silicas, zeolite, alumina, molecular sieves, etc. that can either react with or absorb materials in the fluid stream. Markell et al, U.S. Pat. No. 5,328,758, uses a melt blown thermoplastic web and a sorbtive material in the web for separation processing. Errede et al., U.S. Pat. No. 4,460,642, teach a composite sheet of PTFE that is water swellable and contains hydrophilic absorptive particles. This sheet is useful as a wound dressing, as a material for absorbing and removing non-aqueous solvents or as a separation chromatographic material. Kolpin et al., U.S. Pat. No. 4,429,001, teach a sorbent sheet comprising a melt blown fiber containing super absorbent polymer particles. Deodorizing or air purifying filters are shown in, for example, Mitsutoshi et al., JP 7265640 and Eiichiro et al., JP 10165731.
While both surface loading and depth media have been used in the past and have obtained certain levels of performance, a substantial need remains in the industry for filtration media that can provide new and different performance characteristics than formerly obtained.